CVE-2025-55680: Windows 10 1809 Privilege Escalation Flaw

CVE-2025-55680 Overview

CVE-2025-55680 is a time-of-check time-of-use (TOCTOU) race condition in the Windows Cloud Files Mini Filter Driver (cldflt.sys). An authorized local attacker can exploit the race window between validation and use of a resource to elevate privileges on the affected host. Microsoft published the advisory on October 14, 2025, and the flaw is tracked under CWE-367.

The issue affects a broad range of Windows client and server releases, including Windows 10, Windows 11, and Windows Server 2019 through 2025. Successful exploitation yields high impact to confidentiality, integrity, and availability.

Critical Impact
A local, authenticated attacker who wins the race can execute code in kernel context, resulting in full privilege escalation on the affected Windows system.

Affected Products

Microsoft Windows 10 (1809, 21H2, 22H2)
Microsoft Windows 11 (22H2, 23H2, 24H2, 25H2)
Microsoft Windows Server 2019, 2022, 2022 23H2, and 2025

Discovery Timeline

2025-10-14 - CVE-2025-55680 published to NVD
2025-10-14 - Microsoft releases security update guidance for CVE-2025-55680
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-55680

Vulnerability Analysis

The Cloud Files Mini Filter Driver mediates file system operations for cloud-backed placeholder files used by providers such as OneDrive. The driver performs security or state checks on a resource, then acts on that resource in a later step. Between those two steps, a concurrent thread can modify the underlying object, invalidating the earlier check.

An authenticated local attacker exercises this window to substitute an attacker-controlled object for one the driver previously validated. Because the driver operates in kernel mode, the misuse translates into privileged code execution or arbitrary kernel memory manipulation. The attack requires local access and low privileges, and Microsoft rates attack complexity as high due to the timing precision needed to win the race.

Root Cause

The root cause is a classic TOCTOU flaw ([CWE-367]) in cldflt.sys. The driver does not enforce atomicity between the check and the subsequent operation on a shared file system object. A parallel thread can mutate the object, its metadata, or a reparse point after the driver validates state but before it acts, bypassing intended security constraints.

Attack Vector

Exploitation requires local code execution as a standard user. The attacker spawns concurrent threads: one performs cloud file operations that trigger cldflt.sys code paths, while the second races to swap the target object (for example, replacing a file, directory, or symbolic link) after the driver's check but before its use. Winning the race allows the attacker to escalate to SYSTEM.

No public proof-of-concept exploit or exploitation in the wild has been reported. See the Microsoft Security Update CVE-2025-55680 advisory for vendor-specific technical details.

Detection Methods for CVE-2025-55680

Indicators of Compromise

Unexpected child processes running as NT AUTHORITY\SYSTEM spawned from standard user sessions shortly after cloud file activity.
Anomalous handle activity or repeated open/rename/reparse operations against files under paths managed by the Cloud Files API (for example, OneDrive sync roots).
Kernel crashes or bugchecks referencing cldflt.sys on hosts where users have run untrusted binaries.

Detection Strategies

Hunt for processes that rapidly and repeatedly create, rename, or reparse placeholder files in cloud-synced directories, which is characteristic of TOCTOU race attempts.
Correlate token elevation events (Windows Event ID 4672) with parent processes launched from non-administrative user contexts.
Alert on loading of unsigned or unusual drivers alongside suspicious file system activity targeting cldflt.sys code paths.

Monitoring Recommendations

Enable Sysmon with rules covering process creation, file creation, and image loads on endpoints running OneDrive or other Cloud Files API consumers.
Forward kernel and security logs to a central SIEM and retain enough history to reconstruct short-lived race exploitation attempts.
Track patch state for the October 2025 cumulative updates across all Windows 10, Windows 11, and Windows Server fleets.

How to Mitigate CVE-2025-55680

Immediate Actions Required

Apply the October 2025 Microsoft security updates that address CVE-2025-55680 on all affected Windows 10, Windows 11, and Windows Server systems.
Prioritize patching on multi-user hosts such as Remote Desktop Session Hosts, VDI pools, and developer workstations where local users are most likely to run untrusted code.
Audit local user privileges and remove unnecessary interactive logon rights on servers.

Patch Information

Microsoft has released fixes through the October 2025 Patch Tuesday cycle. Refer to the Microsoft Security Update CVE-2025-55680 advisory for the specific KB articles applicable to each affected Windows build.

Workarounds

No official workaround is published by Microsoft; installing the security update is the supported remediation.
Where patching is delayed, restrict local logon and reduce the number of standard users who can execute arbitrary code on affected hosts.
Consider disabling the Cloud Files (Windows Cloud Files API) feature on servers that do not require cloud sync providers, subject to operational testing.

bash

# Verify the Cloud Files Mini Filter Driver version after patching
sc query cldflt
powershell -Command "Get-Item C:\Windows\System32\drivers\cldflt.sys | Select-Object VersionInfo"